The invention relates to machines for blow-molding plastic products in which a blank of said product is introduced and blow-molded inside a so-called xe2x80x9cwrap-overxe2x80x9d molding unit.
In said machines, the molding units are comprised of two mold supports where each holds a half mold. The supports are hinged in relation to each other in an open position, making it possible to introduce the blank in the cavity defined by the mold, and in a closed position in which the two half-molds rest against each other by their joint face.
In most cases, the hinge axis of the two supports is contained in the plane of the joint face and extends along one edge of the latter.
Such a mold unit is described for example in the patent application GB-A-1.230.090. In said document we see a mechanism that controls the opening and closing of the mold unit. This mechanism comprises a slide that travels perpendicular to the joint axis of the two supports. The slide is linked to each of the supports by a connecting link that is hinged by one extremity on the slide and by its other extremity on the associated support.
Thus, the two connecting links make it possible to convert a straight slide movement into two opposite support rotary movements.
However, to keep the mold unit in the closed position, the proposed mechanism requires the use of specific means.
Indeed, during the blowing, the air under pressure injected in the product presses the latter against the walls of the cavity. Thus, an effort that tends to cause the opening of the mold unit is transmitted over the two parts of the mold. Yet, in the configuration retained in the previously mentioned document, the connecting links are arranged in such a way that, in the closed position, this opening effort results in an effort on the slide that tends to make it move backwards toward its open position.
The value of this effort can become very significant when the blowing air is injected under an intense pressure, for example under a pressure of 40 bars as is common in blow-molding machines for previously injected blanks.
This effort may on the one hand cause the mold to open, but mostly, and very likely, cause a slight detachment of the two parts of the mold at their joint face. Such a detachment is undesirable as it causes the formation on the finished item of an unsightly added thickness.
We know of various systems used to ensure that the mold unit remains in the closed position. It may be a locking system that makes the two mold supports integral by the edge that is opposite their rotation axis. It may also be means of blocking the slide.
In all cases, said locking means lead to, in addition to their actual presence, the presence of control means adapted so as to allow their implementation in synchronism with the machine""s other systems.
Therefore, the objective of the invention is to propose a new control mechanism for the opening and closing of the mold unit that makes it possible to eliminate the presence of specific locking means, even for machines that use a significant blowing pressure.
With this end in view, the invention proposes a machine for blow molding thermoplastic products, of the type in which a mold unit comprises a mold in two parts each carried by a mobile support, of the type in which the two mobile supports are hinged in relation to each other, of the type in which the movements of the two supports, between an open position and a closed position, are controlled by a mechanism that comprises a slide that travels between an opening position and a closing position, and of the type in which each support is connected to the slide by at least one associated connecting link that is hinged on the one hand on the slide and on the other hand on said support, characterized in that, in the closed position, the main axes of the connecting links, that are each determined as being one common perpendicular to the hinge axes of the connecting link on the slide and on the associated support, are oriented in one same direction perpendicular to the hinge axis so that the control mechanism ensures the locking of the molding unit in the closed position.
According to other characteristics of the invention:
in the closed position, the main axes of the connecting links are oriented in one same direction perpendicular to a joint face of the mold parts;
in the closed position, the main axes of the connecting links are oriented in one same direction perpendicular to the translation direction of the slide;
in the closed position, the main axes of two connecting links respectively associated to each of the supports are aligned;
at least one of the two supports comprises several associated connecting links;
the slide comprises at least one stirrup with two arms that extend noticeably in a plane that is perpendicular to the hinge axis of the two supports and that, in the closed position, surround at least partially the two supports, and the connecting links are hinged to the free extremities of the two arms and are solicited during compression to ensure the locking of the mold unit;
the slide comprises two stirrups offset in relation to the axial hinge direction of the two supports, so that each support is associated to at least two connecting links offset in relation to the axial direction;
the slide is controlled by a jack;
the mold unit is integral with a carousel with a continuous rotary movement, and the slide is controlled by a device with a cam and a cam follower;
the slide is solicited by the cam from its closing position towards its opening position, towards an elastic device that continuously solicits it in the direction of its closing position;
the elastic device is comprised of a jack;
during the opening of the mold, the jack exerts a reduced closing effort on the slide;
the jack is a double-acting jack comprising two chambers with a variable volume delimited by a piston and that have different effective sections on the piston so that, when the two chambers are fed with fluid under an identical pressure, one of the chambers exerts on the jack, a force that is greater than that of the other, which tends to solicit the slide toward its closing position; and
the traveling direction of the slide and the hinge axis of the two supports are perpendicular to each other and are contained in a plane that contains the joint face of the mold.